In the prior art there is disclosed sealing glasses having high coefficients of thermal expansion and generally low softening points of about 500.degree. C., such as the lead borosilicate sealing glasses described in U.S. Pat. No. 2,642,633. Also disclosed in the prior art is the method of incorporating ceramics or metal additives into sealing glasses for the purpose of adjusting the high coefficient of thermal expansion of the glasses to a point intermediate that of the materials being sealed. U.S. Pat. No. 3,123,470 discloses incorporating metal particles (molybdenum, copper and iron) into a vitreous bonding material in order to modify the thermal expansion of the seal formed to that of the materials being bonded. U.S. Pat. Nos. 3,258,350 and 3,951,669 likewise disclose incorporating additives (ceramics and glass-ceramics) to sealing glasses for similar reasons.
Seals formed in this manner were generally admixtures of the additive incorporated within a predominantly glassy matrix. A major difficulty with such seals, therefore, was that they were inadequate for use above the softening point of the glass used to form the seal since the glassy seal formed would deform and flow above that temperature. It was thought that firing the seals formed of such admixtures to a temperature sufficient to cause a possible reaction between the frit and the additive would limit the effectiveness of the additive as an expansion modifier.
The invention disclosed herein was designed to solve the specific problem of providing mechanically strong, hermetic seals for bonding an alumina tube to a zirconia tube, used as a solid electrolyte, in oxygen sensor units. Sealants were required which had not only a coefficient of thermal expansion which would match those of the materials being sealed, but which would also remain stable, mechanically strong and impervious to temperatures of 1000.degree. C. and above 1000.degree. C.
The strong bonding of materials, such as ceramics, which experience severe thermally induced expansion-contraction stresses have been difficult to achieve. Generally, in high temperature applications, mechanically strong seals are required to accommodate the dissimilar or substantially different thermal expansions of the bonded materials and thereby diffuse such expansion-contraction stresses.
Cermets are shown in the art as metallic-ceramic materials comprising separately metallic and ceramic phases which are very refractory and hard. U.S. Pat. No. 2,702,750, discloses the forming of cermet materials composed of alumina and aluminum silicon alloy, by reacting aluminum metal with SiO.sub.2 at temperatures of 700.degree. C. to 900.degree. C. The formation of other cermets composed of alumina and aluminum metal alloy is disclosed in U.S. Pat. No. 3,034,908 where a reacting temperature between 900.degree.-950.degree. C. was required. Neither of these patents, however, discloses or suggests reaction temperatures equal to or greater than 1000.degree. C., for the forming of mechanically strong, impervious seals.